Command availability annotations (#9335)

Adds some availability detail to the command line tools recently added
to the core docs

### Motivation:

Saw comments about swift package migrate and swift package show-traits
being in the docs, but not available in released toolchains.

### Modifications:

Added availability annotations to the documents

### Result:

CLI documentation pages will show availability of when commands are
available for versions of the Swift toolchain

Author: heckj

Sources/PackageManagerDocs/Documentation.docc/Package/PackageMigrate.md

@@ -2,6 +2,7 @@
 
 @Metadata {
     @PageImage(purpose: icon, source: command-icon)
+    @Available("Swift", introduced: "6.2")
 }
 
 

Sources/PackageManagerDocs/Documentation.docc/Package/PackageShowTraits.md

@@ -2,6 +2,7 @@
 
 @Metadata {
     @PageImage(purpose: icon, source: command-icon)
+    @Available("Swift", introduced: "6.3")
 }
 
 List the available traits for a package.